This invention relates to automotive (i.e., SLI- and motive power-type) lead-acid storage batteries of the maintenance-free variety which do not require routine water additions while in service. More specifically, this invention relates to the wrought, lead-calcium alloy strip used in the manufacture of grids therefor which are less than about 0.07 in. thick.
Wrought, lead-calcium-tin (i.e., 0.065% Ca, 0.7% Sn) alloys have been conventionally rolled into grid-making strip using high speed tandem mills (i.e., unidirectionally) and subsequently expanded into battery grids according to the process described in Wheadon et al U.S. Pat. No. 3,881,952. It has been said that the calcium and tin content of such alloys can vary between about 0.03% to about 0.10% by weight Ca and between about 0.3% to about 2.0% by weight tin if the tin-to-calcium ratio is at least 7:1 and the alloy is worked before age hardening occurs to yield a stable microstructure resistant to room temperature recrystallization and grain growth, and possessing room temperature stability. Expanded battery grids made from such wrought alloy have demonstrated loss of tensile strength in normal automotive service, especially in the warmer climates, where the battery is consistently exposed to elevated temperatures which promote recrystallization of the alloy and a consequent loss of a significant amount of its tensile strength. This loss in tensile strength promotes early failure of the batteries due to breaking of the grids resulting from the vibration and shock automotive batteries experience. Moreover, such a weakened and recrystallized grid has less resistance to plate growth and is susceptible to catastrophic intergranular corrosive attack.
Accordingly, it is an object of the present invention to provide a process for making strips of wrought, recrystallized, lead-calcium and lead-calcium-tin battery grid alloys which possess stable tensile strength at room temperature as well as at 150.degree. F., and have a reduced susceptibility to catastrophic intergranular corrosion. This and other objects and advantages of the present invention will become more readily apparent from the detailed description thereof which follows.